The orbiting Chandra X-ray telescope has shown that the expansion of the Universe received a kick midway through its life, about six billion years ago.

Its images of clusters of galaxies have provided astronomers with a new way to probe the history of the cosmos.

It casts new light on the unknown "dark energy" that dominates the Universe.

Using the data, astronomers estimate that dark energy makes up about 75% of the Universe, "dark matter" about 21%, and visible matter only about 4%.

'Biggest mystery'

"Dark energy is perhaps the biggest mystery in physics," said Steve Allen, from the Institute of Astronomy at the University of Cambridge, UK, who is the leader of the study.

"As such, it is extremely important to make an independent test of its existence and properties."

Allen and his colleagues used Chandra to study 26 clusters of galaxies at distances corresponding to light travel times of between one and eight billion years.

Chandra is sensitive to X-rays

The data they obtained spans the time when the Universe slowed from its original expansion, before speeding up again.

Many astronomers attribute the driving force behind this cosmic acceleration to the mysterious dark energy - a strange form of energy that acts like repulsive gravity.

It could be due to extra dimensions of space, or it could be an indication that modifications of Einstein's General Theory of Relativity are needed.

The new results indicate the transition from a decelerating to an accelerating phase occurred several billion years ago, and give intriguing clues about the nature of dark energy and the fate of the Universe.

Independent technique

Chandra's observations agree with supernova results, including those from the Hubble Space Telescope, which first demonstrated the cosmic acceleration.

Importantly, Chandra's results are completely independent of the supernova technique.

"Our Chandra method has nothing to do with other techniques, so they're definitely not comparing notes, so to speak," said Robert Schmidt, from the University of Potsdam, Germany.

Better limits on the amount of dark energy in the Universe and how it varies with time can be obtained by combining the X-ray results with data from Nasa's Wilkinson Microwave Anisotropy Probe and its observations of the cosmic microwave background radiation.

Using the combined data, Allen and his colleagues found that dark energy makes up about three-quarters of the Universe.

These results will appear in an forthcoming issue of the Monthly Notices of the Royal Astronomy Society.